Temperature-dependent Isomerization Research Articles

Phase transition of spiropyrans: impact of isomerization dynamics at high temperatures.

Isomerization behaviors of spiropyran derivatives in neat condensed phase were studied to understand their unusual phase transitions including cold-crystallization after extreme supercooling down to -50 °C. Compounds with different functional groups were compared, and the equilibrium between isomers at high temperatures was found to determine phase transitions.

Tanning predicts bone mass but not structure in adolescent females living in Hawaii

To evaluate the relationship between facultative skin pigmentation, which predicts circulating levels of plasma 25-hydroxymitamin D, and several measures of bone mass and structure in a cross sectional sample of adolescent females living in Hawaii. Our sample was composed of adolescent females (n = 94) living in Hawaii where seasonal sun exposure is minimal, and who self-identified as either white (n = 16) or Asian (n = 78). Bone mineral content (BMC) of the total body, the lumbar spine and the hip, and cross sectional area (CSA) and section modulus (Z) at the proximal femur were quantified using DXA. Facultative skin pigmentation was measured at the forehead and non-facultative skin pigmentation was measured at the inner arm using a Chroma Meter CR-200b colorimeter. There were no significant differences between ethnic groups in terms of skin pigmentation. The difference between a* taken at the forehead and inner upper arm significantly predicted BMC at the lumbar spine, total hip, and total body. Other measures of skin pigmentation were not significant predictors of any other measure of skeletal integrity. The difference between facultative and non-facultative skin pigmentation for a* is a significant predictor BMC, but not bone structure. Our findings are limited by an inability to control for long term UVA and UVB exposure and lack of a measure of serum 25(OH)D status. Further research is needed to examine these questions, particularly in populations who live at high latitudes where a winter season limits vitamin D(3) synthesis.

Effects of thermal processing on trans– cis-isomerization of β-carotene in carrot juices and carotene-containing preparations

The effects of thermal processing (blanching, pasteurization, sterilization) on trans–cis-isomerization of β-carotene in carrot juice produced on pilot plant scale and in β-carotene-containing preparations (bovine serum albumine and lecithine suspensions) were studied. While pasteurization and sterilization at 121 °C caused only minor isomerization, sterilization at 130 °C and blanching resulted in increased levels of cis-isomers. Dissolution of crystalline carotenes by cellular lipids during blanching of carrots was identified as the prerequisite for isomerization. Also addition of grape seed oil to the coarse mash enhanced isomerization both in unheated and heat-preserved juices. Model preparations containing crystalline β-carotene showed pronounced stability during heating, whereas thermal treatment of β-carotene dissolved in toluene resulted in temperature-dependent isomerization. UV/vis spectroscopy of carotene-containing particles obtained by isopycnic density-gradient centrifugation of carrot juice indicated the presence of crystalline carotene. The carotene-stabilizing principles present in carrot juice are assumed to consist of chromoplast fragments as the core coated by water-insoluble juice constituents such as pectin, cellulose and protein which prevent carotenes from being dissolved in neutral lipids upon moderate heating.

Spectroscopic and Thermodynamic Characterization of Strontium Carbonyls Formed upon Carbon Monoxide Adsorption on the Zeolite Sr−Y

The adsorption of CO on the faujasite-type zeolite Sr−Y generates four types of carbonyls: the Sr(CO)2+ species, which has a characteristic IR absorption band at 2191 cm-1; the Sr(OC)2+ species at 2095 cm-1; Sr(CO)22+, which has a single mode at 2187 cm-1; and Sr(CO, OC)2+, which has a mode at 2098 cm-1. The use of a variable-temperature IR cell allowing the simultaneous measurements of temperature, pressure, and coverages (intensities) of CO species to be made has allowed the determination of the standard enthalpy and entropy of formation of each species, so obtaining a fairly thorough thermodynamic characterization of the CO/Sr−Y system. C- and O-bonded carbonyl species were found to be in a temperature-dependent isomerization equilibrium, which was also studied by means of variable-temperature IR spectroscopy.

Isomerization and Reactivity of Allyl Groups Adsorbed on Coinage Metals

Using allyl halides (C3H5X, X = Cl, Br) to generate adsorbed C3H5 fragments and halogen atoms, we have studied the isomerization and reactivity of σ-bonded (η1) and π-bonded (η3) C3H5 on two metal substrates, Cu(100) and Ag(111), using vibrational and thermal desorption spectroscopies. On Cu(100), two forms of η3-C3H5, denoted endo-η3-C3H5 ↔ exo-η3-C3H5, are identified that undergo a reversible temperature-dependent isomerization. When dissociative adsorption of C3H5X saturates on Cu(100), only exo-η3-C3H5 is found between 77 and 300 K. On Ag(111), η1-C3H5 and η3-C3H5 (both exo and endo) form at 110 K, depending on the dose. The η1-C3H5 and exo-η3-C3H5 irreversibly isomerize to endo-η3-C3H5 upon annealing to 175 K. When either metal substrate, held at a selected temperature between 77 and 150 K, is dosed with C3H5Br, there is a reaction with η3-C3H5 to form 1,5-hexadiene, C6H10. This reaction does not occur upon dosing with C3H5Cl. When η3-C3H5 is heated above 200 K, different results obtain for the two m...

Electronic Relaxation and Ground-State Dynamics of 1,3-Cyclohexadiene andcis-Hexatriene in Ethanol

The transient absorption induced by UV excitation of 1,3-cyclohexadiene and Z-hexatriene dissolved in ethanol was measured in the wavelength range 255−450 nm. Repopulation of the ground state takes 470 fs in tZt-hexatriene. The same process in 1,3-cyclohexadiene, accompanied by ring opening to Z-hexatriene, occurs in less than 300 fs. The analysis of the long-wavelength wings of the ground-state spectra reveals a cooling time constant of 7 ps for the hot product Z-hexatriene. The transient absorption in the UV furthermore reflects the single-bond isomerizations of this product to different conformers. The evolution of their concentrations was simulated by rate equations using temperature-dependent isomerization rates calculated with the Arrhenius law. In this model, the concentrations of the conformers reach thermal equilibrium in a few picoseconds; during the first 10−20 ps, the concentrations follow the cooling of the molecule by the solvent, staying near thermal equilibrium; and a small quantity of cZt-hexatriene is trapped in its potential well on a time scale of 100 ps at the final temperature (300 K). At this temperature, equilibration takes longer. This model well reproduces the time dependence of the integrated spectra in solution and recent experimental results in gas phase. In addition it allows a consistent interpretation of previous, time-resolved resonance−Raman experiments. The fact that single-bond isomerization in the hot product was not suppressed by contact with the cold solvent suggests that this could also be the case for photorhodopsin, the postulated primary conformer of the photoproduct of rhodopsin which always eluded trapping even at cryogenic temperatures.

A mutant RNA polymerase that forms unusual open promoter complexes.

We describe a mutant Escherichia coli RNA polymerase (RNAP) that forms stable open promoter complexes even at -20 degrees C but with a shortened melted region that extends downstream to only position -7. In the presence of initiating transcription substrates, the mutant RNAP undergoes a temperature-dependent isomerization, resulting in a promoter complex that is indistinguishable from the wild-type RNAP-promoter complex, with the melted region extended downstream to position +4. We propose that the open complex formed by the mutant RNAP represents an intermediate on the normal promoter-opening pathway and that our results support earlier findings that initial promoter opening occurs in the upstream region of the -10 promoter consensus element and subsequently extends downstream to encompass the transcription start site.

Conformational Isomerization of 1,2-Difluoroethane in Solid Argon: Cage-Modified Reaction Barriers

Thermally induced rotational isomerization of trans-1,2-difluoroethane into the more stable gauche conformer in Ar matrices was studied over the temperature range 30−36 K by infrared spectroscopy. Photochemical generation of 1,2-difluoroethane in the matrix by two different mechanisms forms the product in singly and doubly substitutional argon lattice sites. Only the more stable doubly substitutional site is populated by the ordinary vapor deposition technique. At 33 K, the first-order rate constant for isomerization in the single site is 1.0 × 10-5 s-1 whereas the corresponding value for the double site is 5 × 10-3 s-1. Measurements of the temperature-dependent isomerization rates show that the activation energies for molecules in single and double sites are 15.0 ± 1.5 and 9.7 ± 0.8 kJ/mol, respectively. The latter value is close to a recent estimate of the gas phase reaction barrier (8.86 kJ/mol). This study provides an important example of how a reaction barrier is significantly modified by a solid-sta...

Kinetic matrix effect on photochromic reactions of spirobenzopyran dispersed in or covalently bound to poly(alkyl methacrylate) matrices.

Spirobenzopyran (SP) was dispersed (DP) in or covalently bound (CB) to poly(methyl methacrylate) (PMMA), poly(ethyl methacrylate) (PEMA), and poly (isopropyl methacrylate) (PiPMA) matrices. The extent of coloration due to formation of a merocyanine was smaller in CB than in DP matrices on irradiation of a single pulse of a Nd: YAG laser (355nm). Photochemical isomerization from a merocyanine to a SP obeyed the first-order kinetics above Tg of the matrix polymer, but not below Tg. The isomerization below Tg was analyzed on the basis of the kinetic matrix effect, by assuming a temperature-independent fast isomerization and a temperature-dependent slow isomerization at lower T than 240K, but only a temperature-dependent isomerization at higher T than 240K. The matrix response function time constant, α, for the fast isomerization was larger in CB than in DP matrices. Arrhenius plots of α for the slow isomerization showed discontinuities at characteristic temperatures of matrix polymers.

Lewis‐Säure katalysierte Additionen an ungesättigte Fettstoffe I: SnCl4‐katalysierte Addition von Nitrilen an Ölsäuremethylester und 10‐Undecensäureethylester

AbstractNitrile wurden in einer SnCl4‐katalysierten Ritter‐Reaktion an die Ester ungesättigter Fettsäuren addiert unter Bildung von N‐Acylaminofettsäureestern in guter Ausbeute und unter relativ geringer, temperaturabhängiger Isomerisierung.

Facile geometric isomerization of phenolic non-steroidal estrogens and antiestrogens: Limitations to the interpretation of experiments characterizing the activity of individual isomers

In many estrogen responsive systems the isomers of tamoxifen are known to have different biological character-the trans isomer is generally an antagonist and the cis isomer an agonist. Attempts to similarly characterize the isomers of hydroxytamoxifen (which differ greatly in their affinity for the estrogen receptor) are shown to be complicated by their facile isomerization. This isomerization was studied in cultures of estrogen receptor positive MCF-7 human breast cancer cells and monitored by HPLC under reversed phase conditions. Hydroxytamoxifen isomers that are initially 99% pure, undergo a time and temperature dependent isomerization, so that after 2 days in tissue culture medium at 37°C they have isomerized to the extent of 20%. This isomerization occurs in the cell-free medium alone and cannot be attributed to a metabolic conversion by the cells. The isomerization occurs much more slowly at 4 than at 37° C and can be reduced considerably by various antioxidants (butylated hydroxytoluene, ascorbate, α-tocopherol, retinoic acid and retinal); however, at concentrations that block isomerization, these antioxidants are toxic to the cells. Although the medium contains both the cis and trans isomers of hydroxytamoxifen, the MCF-7 cells preferentially accumulate the trans isomer and the material associated with the nuclear estrogen receptor is, in all cases, mainly the higher affinity trans isomer. A similar preference of the estrogen receptor for the trans isomer is seen with diethylstilbestrol, resulting again in almost exclusive accumulation of the trans isomer in the receptor binding site. These experiments indicate the importance of verifying the isomer compositions of easily isomerizable non-steroidal estrogens and antiestrogens, such as diethylstilbestrol and hydroxytamoxifen, both in stock solutions and in experimental samples (especially those derived from receptor-associated material), so as to ascertain that the activity of the individual isomers is being correctly assigned.

The Cutaneous Photosynthesis of Previtamin D3: A Unique Photoendocrine System

The skin has been recognized as the site for the sun-mediated photosynthesis of vitamin D3; until recently, however, very little was known about either the sequence of events leading to the formation of vitamin D3 in human skin or the factors that regulate the synthesis of this hormone. It is now established that, during exposure to sunlight, the cutaneous reservoir of 7-dehydrocholesterol (principally in the stratum Malpighii) converts to previtamin D3. Once this thermally labile previtamin is formed, it undergoes a temperature-dependent isomerization to vitamin D3 over a period of 3 days. The plasma vitamin-D binding protein preferentially translocates vitamin D3 from the skin into the circulation. During prolonged exposure to the sun, the accumulation of previtamin D3 is limited to about 10 to 15% of the original 7-dehydrocholesterol content because the previtamin photoisomerizes to 2 biologically inert photoproducts, lumisterol3 and tachysterol3. Increases in either latitude or the melanin concentration in the skin diminish the epidermal synthesis of previtamin D3. A single total body exposure to 3 minimal erythemal doses of ultraviolet radiation increased the vitamin-D3 levels in the serum 25-hydroxyvitamin-D levels after 7 days. The unique mechanism for the cutaneous synthesis, storage, and steady release of vitamin D3 into the circulation prompted an investigation into the potential therapeutic benefits of using the skin as the site for the synthesis and absorption of vitamin-D3 metabolites.

An investigation of heavy meromyosin-ADP binding equilibria by proton release measurements.

The interaction of magnesium-ADP with skeletal muscle heavy meromyosin has been studied by measuring the accompanying release of protons. Total pH changes of the order of 0.03 were involved, and measurements were performed with a discrimination of some ten-thousandths of a pH unit. At pH 8.0 and 25 degrees C about 0.5 mol of protons per mol of heavy meromyosin is released at saturation. A stoichiometry of binding close to 2 mol of ADP per mol of protein was found, with a binding constant, obtained from the proton release titration curve (pH 8.0, 25 degrees C), of 2 X 10(5) M-1. At 5 degrees C the release of protons per mole is slightly greater, and the binding constant is somewhat increased, reflecting a negative enthalpy of binding. Similar proton release behavior is observed in the presence of manganous ions in place of magnesium. The liberation of protons is thus unrelated to the temperature-dependent isomerization of myosin in the presence of substrate. Alkylation of a reactive thiol group (SH1) does not change the proton liberation at pH 8.0. From the pH dependence of proton release, the association constant of heavy meromyosin with magnesium-ADP at other pH values can be inferred and shows an appreciable rise as the pH increases. The pH-proton release profile also allows the pK of the ionizing groups perturbed by the ligand to be deduced. At least two groups ionizing above pH 7 and one below are involved. Their pK's in the unperturbed state are assigned as 8.5, 9.3, and about 6.6, respectively; they are displaced in the complex to about 8.0, 9.1, and 6.3. A relation to the pH-activity profile of myosin ATPase is indicated. The pH-proton release profile is somewhat changed when the SH1 group is alkylated. Measurements with potassium-ADP, in the absence of magnesium, show that at pH 8.0 there is no proton release but rather a sizeable proton absorption (about 0.5 mol of protons per mol of heavy meromyosin). The association constant derived from the titration curves (pH 8.0, 25 degrees C) is 3 X 10(4) M-1.